1. Technical Field of the Invention
The present invention relates to a flowmeter for detecting flow rate in a pipe or a tube. The present invention relates more particularly to a vortex flowmeter.
2. Description of Related Art
Since flow rate is an primary variable in fluid dynamics, how to measure flow rate in a pipe or a tunnel, which is called an inner flow, becomes important in scientific researches and industrial applications.
Plural methods can be used to measure the flow rate in different conditions. As a most simple example, in a fully developed laminar pipe flow, we can easily get the measurement of flow rate after we had determined the velocity at a point and the location of this point and the diameter of the pipe. Besides, several commercially available flowmeters are briefly discussed below.
Orifice meters have been by far the most popular for years. The principle behind orifice meters is that, a differential pressure transmitter measures pressure drop across a restriction in the line. The restriction is usually a concentric orifice, with the orifice diameter being 10% to 75% of the inside pipe diameter. The pressure drop can be measured with flange taps. The simplified equation, EQU F=C P
where F=volumetric flow rate, P=differential-pressure measurement, and C orifice coefficient, represents the relations between flow rate and pressure difference.
The orifice meter provides a time-proven and relatively low-cost approach for measuring most flows. But, the orifice meter does have its limitations, in that under certain situations, for various reasons, it will not work. For liquid service, these cases include systems where: the necessary pressure drop in not available (in most gravity-flow applications); the fluid will flash at the reduced pressure that occurs in the throat of the orifice meter; or flow rate is high.
About the orifice meter, since flow is related to the square root of the pressure drop, it requires a square-root extractor between the output of the differential pressure transmitter and a flow controller in order to give a linear relationship, which is at the sacrifice of cost and accuracy.
Other differential pressure type meters include: venturi meter, flow nozzle, annular orifice gentile tube, wedge meter, integral orifice, pitot tube, elbow meter, variable area meter, target meter, sonic nozzle, multi-port pitot, dall tube, variable aperture meter, etc.
Positive displacement meters operated by using mechanical divisions to successively displace discrete volumes of fluid. This principle of operation is essentially simple, but the accuracy depends upon precision in both manufacture and assembly.
A common used positive displacement meter is the rotary piston meter. This employs a cylindrical piston which is displaced around a cylindrical chamber by the flowing liquid. Rotation of the piston drives an output shaft which is used to operate counters. Rotary piston meters can handle a wide range of process liquids with a large range of viscosity.
Other positive displacement type meters include: reciprocating piston, sliding vane, nutating disc, oval gear, helix meter, bi- and tri-rotors, metering pump, roots blower, diaphragm meter, wet gas meter, bellows meter, etc.
The principle of operation of inferential type meters uses a rotating component (wheel, vane, rotor or helical runner) to convert free stream energy into rotary motion. This rotary motion is then detected by some type of pick-up device, i.e. magnetic, optical, radio frequency, or gears and a mechanical counter. Axial turbines are the most widely available and accurate type of inferential type meters.
Vortex shedding meter is the most commonly used among fluid oscillatory type meters. A vortex flowmeter detects frequency of vortex shedding from a bluff body which, as learned from fluid dynamics, is linearly proportional to the fluid velocity under certain conditions. Relevant papers discussing this phenomenon can be found in: H. V. Mangin, Tappi 58, 65 (1975); D. J. Lomax, Control Instrument 7, 36 (1975); and T. J. S. Brain and R. W. W Scott, J. Phys. E 15, 967 (1982).
Conventional vortex flowmeter is a two dimensional bluff body accommodating a pressure sensor or a probe for detecting vortex shedding frequencies. The bluff body is across the flow field with its front face facing the stream as shown in FIG. 1A and 1B. The upper edge and lower edge of the front face form sharp angles with the front face. As fluid flows under certain conditions, vortices alternatively develop behind both edges of the bluff body. The shedding frequency is defined as the number of vortices developed in a certain time. Since shedding frequency is linearly proportional to the flow speed, flow speed can be easily evaluated. The accuracy of vortex flowmeter is relatively high in a wide range of flow speed. The turndown ratio, which is the maximum measurable flow rate over the minimum measurable flow rate, is up to 100 to 1, at an accuracy of 0.5% of meter reading.
The vortex flowmeter has been well accepted as a competitor for accurate and inexpensive flow rate measurement. Current vortex flowmeters are designed with a two-dimensional-like bluff body together with a sensor that is either integrated into the bluff body or separately situated downstream for detecting the vortex shedding frequency, as described in M. Takamoto and K. Komiya, J. Fluid Control 11, 27 (1979), and P. G. Scott, "Use of Vortex Flowmeters for Gas Measurements," J. Petroleum Technol. 33, 2082 (1981).
It is considered that the maintenance and replacement of pressure sensor is not quite easy. Pressure sensor could not be detached from the bluff body unless the pipe is disconnected. This is due to the limitation of the design of two dimensional bluff body because pressure sensor must be placed behind the bluff body. In an experiment, one can not know whether the probe works precisely or not. Even he (she) knows the error arises from the probe, changing the probe may change the structure of flow field.
Other types of flowmeters not described above includes: electromagnetic meters, ultrasonic meters, mass meters, thermal meters, miscellaneous meters, solids meters, open channel meters, etc.